1. Field of the Invention
The present invention relates to apparatus useful in transferring fluids and in particular, the present invention relates to piggable fluid swivels which can be used for example, to bypass fluid lines around the universal joints of production risers.
2. Description of the Prior Art
During the past several years the search for oil and gas in offshore areas has greatly expanded and has now progressed into deep waters such as those found in the North Sea. To facilitate production of oil and gas from remotely located offshore fields, complex tanker mooring systems, serving as a centralized production site for the entire field, have been developed. Normally, a production riser having one or more hard fluid lines extending from a subsea location along the length of the riser to the mooring site permit the transfer of fluids between the subsea location and a tanker moored to the mooring site. Certain fluid lines may be used to convey oil and gas from the subsea location into the tanker while other fluid lines may be used to inject liquids or gases back into the field for the purpose of well stimulation or storage.
Under the influence of wind and waves, the riser will tend to move in various directions about its axis. To permit the riser to move freely without breaking, the riser is normally provided with one or more universal joints along its length. The hard fluid lines associated with the riser are normally provided with a plurality of fluid swivels to bypass the lines around the universal joints. Normally, in most sea states, riser movement is minimal and the fluid swivels do not pivot more than about .+-.15.degree..
Many crude oils contain solid materials, such as wax or asphalt, which stick to the walls of the fluid lines and fluid swivels. With use, these solid materials are deposited on the walls of the fluid lines and swivels. These deposits reduce the effective internal diameter of the lines and swivels and impede the flow of fluid therethrough even to the point of total blockage. It is customary in the oil industry to force a scraping device known as a pig through pipelines to remove solids deposited on the inside walls. A pig generally is a spherical, cylindrical or ellipsoidal shaped and is selected to have a circumferential dimension equal to or slightly greater than the circumferential dimension of the pipeline. It is normally necessary to periodically force a pig through pipelines to clean the pipelines.
Prior piggable fluid swivels are disclosed in U.S. Pat. Nos. 4,174,127 and 4,183,559. U.S. Pat. No. 4,174,127 discloses a swivel comprising generally a stationary shaft and a rotatable housing member circumferentially mounted thereon. The swivel also has a circumferential pig raceway and a pig deflector. The pig passes from a first fluid line extending through the shaft into the swivel raceway. The deflector catches the pig and moves it through the housing member and out of the swivel into a second fluid line. Unfortunately, the swivel can only be pigged in one direction. Further the diameter of the swivel must be increased substantially to accommodate the raceway. The increased diameter causes a substantial increase in the overall weight and size of the swivel and presents greater handling problems for the swivel. U.S. Pat. No. 4,183,559 discloses a similar type of swivel having circumferential raceways formed in a spiral. Unfortunately the overlapping raceways cause turbulent fluid flow through the swivel. Thus, there is a need in the art for a relatively light weight improved piggable fluid swivel.